A tubular plastic foil of the type used to make bags is formed by extruding a tubular plastic strand from an annular mouth of a nozzle. A stabilizer extends centrally from this nozzle and has an open outer end. Cooling air is directed radially inward at the strand as it exits from the nozzle and is also injected into the annular space between the strand and the stabilizer. The cooling air trapped in the strand serves both to expand and cool it, and is aspirated at some longitudinal spacing from the nozzle into the stabilizer.
This arrangement is typically oriented vertically with the strand moving up and away from the nozzle. After being expanded by air pressure inside it, the tube is flattened and then is usually cross-welded and perforated or cut into bags.
The stabilizer is typically of circular section and has a closed outer end and perforations at this outer end through which the cooling air is aspirated. Such an arrangement is satisfactory so long as the neck part of the strand, that is the strand before it is radially expanded to the desired size, is relatively short. When, however, high-pressure polyethylene is being extruded this neck is fairly long, so that it is fairly common for the resin to come into contact with the intake end of the stabilizer tube and become caught, forcing one to shut down the machine and clear the jam.
It has been suggested to avoid this problem by providing along the stream of cooling air flowing along the outside of the strand and spaced from this strand a sleeve which is provided over its length with spaced circularly annular disks which extend into the flow of the outer stream of cooling air. Such a system is only a poor partial solution to the problem in that jams still occur.
In U.S. Pat. No. 4,955,802 the apparatus has a nozzle having an annular mouth generally centered on a longitudinal axis for longitudinally outwardly extruding a tubular plastic strand, an inner stabilizing tube extending longitudinally outward from the nozzle within the mouth thereof and having an outer end spaced a predetermined longitudinal distance from the mouth, and an outer stabilizing tube surrounding the inner tube and defining a longitudinal passage therewith, extending longitudinally outward from the nozzle within the mouth thereof, and having an outer end lying between the outer end of the inner tube and the mouth. The passage opens outward between the outer ends. Cool air is fed to the passage at the mouth and to a space defined between the strand where it emerges from the mouth and the outer tube for flow of this cooling air longitudinally outward between the strand and the outer tube and from the outer end of the outer tube. Thus the strand is expanded transversely generally at the outer ends of the tubes. The cooling air is withdrawn from within the strand by drawing this cooling air into the outer end of the inner tube and longitudinally backward through the inner tube.
Similarly in U.S. Pat. No. 5,322,431 this problem is at least partially solved by distributing over the length of the device formed by the inner tube and the outer jacket or tubular member, between the outlet at the foot side and the outlet at the head side, at least one blowing air intake device at which the blowing air is withdrawn from the flow passage formed by the clearance between the jacket of the device and the thermoplastic tube, into the inner tube, and toward the head of the device, but spaced from the outlet at the head side. At least one further blowing air outlet for feeding the blowing air into the flow passage is provided between the jacket and the thermoplastic tube. The intake as well as the additional blowing-air outlet are aerodynamically matched with respect to flows therethrough to stabilize the plastic tube and/or the film balloon.
Both such arrangements are fairly efficient but could stand some improvement.